1. Field of the Invention
The present invention relates to a plasma display panel, and more particularly, to a method for resetting a plasma display panel which can improve contrast, and apparatus thereof.
2. Description of the Background Art
Recently, a plasma display panel (hereinafter, referred to as “PDP”) that can be easily fabricated as a large-scale panel has attracted public attention as a flat panel display device. The PDP is adapted to display an image by controlling a gas discharge period of each of pixels according to digital video data. FIG. 1 is a perspective view showing the structure of a discharge cell in a conventional plasma display panel. A representative PDP is one having a three-electrode and driven as an AC voltage, as shown in FIG. 1.
A discharge cell of an AC type PDP shown in FIG. 1 includes a pair of sustain electrodes 12A and 12B formed on the bottom of an upper substrate 10, and a data electrodes 20 formed on the top of a lower substrate 18.
Each of the pair of the sustain electrodes 12A and 12B includes a dual layer structure of a transparent electrode and a metal electrode. These pair of the sustain electrodes 12A and 12B consist of the scan electrode 12A and the sustain electrode 12B. The scan electrode 12A mainly supplies a scan signal for an address discharge and a sustain signal for a sustain discharge. The sustain electrode 12B mainly supplies a sustain signal, while operating in turn with the scan electrode 12A. The data electrodes 20 is formed to intersect the pair of the sustain electrodes 12A and 12B and supplies a data signal for the address discharge.
An upper dielectric layer 14 and a protection film 16 are laminated on the upper substrate 10 on which the pair of the sustain electrodes 12A and 12B are formed. A lower dielectric layer 22 is formed on the lower substrate 18 having the data electrodes 20 formed thereon. The upper dielectric layer 14 and the lower dielectric layer 22 serve to accumulate electric charges generated by a discharge. The protection film 16 serves to prevent the upper dielectric layer 14 from being damaged due to sputtering of plasma particles and increase efficiency of secondary electron emission, upon discharge. These dielectric layers 14 and 22 and the protection film 16 cause a driving voltage supplied from the outside to be lowered.
Barrier ribs 24 are formed at the lower substrate 18 on which the lower dielectric layer 22 is formed. A phosphor layer 26 is formed on the surfaces of the lower dielectric layer 22 and the barrier ribs 24. The barrier ribs 24 serve to separate discharge spaces and to prevent the ultraviolet rays generated by a gas discharge from leaking toward neighboring discharge spaces. The phosphor layer 26 is light-emitted by the ultraviolet rays generated by the gas discharge, producing a red (hereinafter, referred to as as “B”) visible rays. Furthermore, the discharge spaces are filled with insert gases for the gas discharge.
This discharge cell is selected by an address discharge due to the data electrodes 20 and the scan electrode 12A, and the selected discharge cell maintains its discharge by means of a sustain discharge due to the pair of the sustain electrodes 12A and 12B. Also, the discharge cell enables the phosphor to emit light by means of the ultraviolet rays generated during the sustain discharge, thus producing the R, G or B visible ray. In this case, the discharge cell implements a gray scale that is necessary to display an image by controlling a sustain discharge period, i.e., the number of a sustain discharge depending on the video data. Moreover, three discharge cells on which the R, G and B phosphors are covered are combined to implement color of one pixel.
FIG. 2 shows the configuration of sub-fields included in one frame. A typical method for driving this PDP is an ADS (Address and Display Separation) driving method wherein driving is performed with a period divided into an address period and a display period, i.e., a sustain period separately. In the ADS driving method, one frame 1F is divided into a plurality of sub-fields SF1 to SF8 corresponding to respective bits of the video data, as shown in FIG. 2. Each of the sub-fields SF1 to SF8 is then divided into a reset period RPD for initializing a discharge cell, an address period APD for selecting a discharge cell, and a sustain period SPD for maintaining discharge of the selected discharge cell. In the above, different numbers of sustain pulses by the sub-fields SF1 to SF8 are assigned to the sustain period SPD, and the sustain period SPD is assembled according to the video data, whereby the PDP implements a corresponding gray scale.
FIG. 3 shows a driving waveform in a conventional plasma display panel.
Referring to FIG. 3, each of the first and second sub-fields SF1 and SF2 includes a reset period RPD for initializing discharge cells, an address period APD for selecting discharge cells, a sustain period SPD for maintaining discharge of the selected discharge cell, and an erasing period EPD for discharge erasing.
FIG. 4 shows a process in which wall charges are changed in a reset period. The reset period RDP includes a set-up period SUPD for forming wall charges in all the discharge cells, and a set-down period SDPD for erasing unnecessary wall charges from the discharge cells. In the set-up period SUPD, a ramp-up pulse RUP where a voltage slowly rises from a sustain voltage Vs to the peak voltage Vp is supplied to the scan electrodes Y. A reset discharge occurs in all the discharge cells by means of the ramp-up pulse RUP. Accordingly, wall charges of the negative polarity are formed on the side of the scan electrodes Y and wall charges of the positive polarity are formed on the side of the sustain electrodes Z and the data electrodes X, as shown in FIG. 4.
Thereafter, in the set-down period SDPD, a ramp-down pulse RDP where a voltage of the scan electrodes Y drops from the peak voltage Vp to the sustain voltage Vs and a voltage slowly drops from the sustain voltage Vs to the ground voltage is supplied. Since a weak erasing discharge occurs in all the discharge cells by means of the ramp-down pulse RDP, unnecessary wall charges are erased and wall charges required in a subsequent address discharge remain, as shown in FIG. 4.
Meanwhile, in the set-up period SUPD, the ground voltage is applied to the sustain electrodes Z and the data electrodes X. In the set-down period SDPD, a DC bias voltage BP of the positive polarity is applied to the sustain electrodes Z and the ground voltage is applied to the data electrodes X.
In the address period APD, the scan pulse SP of the negative polarity is sequentially applied to the scan electrodes Y and the data pulse DP of the positive polarity is applied to the data electrodes X in synchronism with the scan pulse SP. Accordingly, in a corresponding discharge cell, a voltage difference between the scan pulse SP and the data pulse DP and a wall voltage by means of the wall charges generated in the reset period RPD are added. Thus an address discharge occurs. By means of this address discharge, wall charges to be used in a subsequent sustain discharge are formed within the corresponding discharge cell. In this address period APD, the DC bias voltage BP is supplied to the sustain electrodes Z.
In the sustain period SPD, sustain pulses SUSPy and SUSPz are alternately applied to the scan electrodes Y and the sustain electrodes Z. Therefore, in the discharge cells in which the wall charges are formed by the address discharge, the wall voltage and each voltage of the sustain pulses SUSPy and SUSPz are added. Thus, whenever the sustain pulses SUSPy and SUSPz are applied, the sustain discharge occurs. By means of this sustain discharge, a corresponding discharge cell emits a visible ray proportional to the sustain period SPD.
In the erasing period EPD, the erase pulse SP is applied to the sustain electrodes Z and an erasing discharge thus occurs. Therefore, wall charges within the discharge cell are erased.
As such, in the conventional method for driving the PDP, the reset period RPD is required every sub-field in order to form wall charges to be used in the address period APD. In the reset period RPD, however, unnecessary light is generated due to the reset discharge generated in all the discharge cells. Therefore, there is a problem that contrast is degraded.
In the concrete, during the set-up period SUPD of the reset period RPD, the reset discharge occurs between the scan electrodes Y and the sustain electrodes Z and between the scan electrodes Y and the data electrodes X by means of the ramp-up pulse RUP supplied to the scan electrodes Y. Discharge that degrades contrast in this reset discharge is a surface discharge between the scan electrodes Y and the sustain electrodes Z. This is because light generated by the surface discharge between the scan electrodes Y and the sustain electrodes Z is generated in the whole area of the discharge cell. Therefore, in order to reduce unnecessary light occurring in the set-up period SUPD, it is required that the discharge between the scan electrodes Y and the sustain electrodes Z be small and short.